This invention relates to smoke detectors, and more particularly to smoke detectors capable of compensating for moisture.
Smoke detectors are commonly used to detect the presence of smoke particles in the air by sensing light scattered from a light beam by smoke particles that infiltrate the smoke detector. In particular, the smoke detector typically includes a housing that defines a chamber that allows smoke to enter without allowing light to enter from the outside. A light source, such as a light emitting diode (LED), is disposed within the chamber for emitting light. A detector, such as a photoelectric eye or photodiode, is also disposed within the chamber. In the absence of smoke, most of the light emitted by the light source is typically absorbed by the chamber walls or some other light trap prior to reaching the detector. In this regard, the walls of the chamber are typically painted a dark color, such as flat black, in order to absorb most of the light incident thereupon. If smoke is present within the chamber, however, the light is scattered by the smoke particles, and a portion of the scattered light is received by the detector, which can cause an alarm if the incident light exceeds a predetermined limit that is indicative of an undesirable concentration of smoke.
In airplane applications, a test of the smoke detector, typically conducted by built-in-test equipment (BITE), must be made before each flight. These detectors typically use the diffusely scattered light from the walls of the chamber to test whether the system is working. In this way, maintenance personnel and/or flight crews can verify that both the light source and the scattered light detector are operational before a flight. Typically, the pre-flight check includes switching on the light source in the chamber and measuring the small level of light scattered from the walls of the chamber that is incident upon the detector. The detector must be sensitive to slight changes in the reflected light inside the chamber in order to detect smoke precisely. Thus, the detector is typically designed to signal a fault signal or an alarm if the reflected light received by the detector is above or below a predetermined range.
Unfortunately, conventional smoke detectors do not behave as expected in the presence of moisture within the chamber. In particular, the light otherwise scattered throughout the chamber by reflections from the walls is dramatically reduced if the walls of the chamber become wet, such as by condensation or humidity caused by the cargo or atmospheric changes. The cause of this behavior primarily lies in the composition of paint. More specifically, paint is typically composed of a clear or substantially clear medium and a pigment, which comprises a finely divided powder. On a close level, flat paint has a surface that is not smooth. Instead, the surface is composed of a large number of flat facets that are randomly oriented and which scatter incident light into a hemispherical pattern. The amount of scattered light from such a reflector is referred to as the Fresnel reflection and is roughly 4% at normal incidence from materials with an index of refraction of about 1.5, which is typical in conventional smoke detectors.
As known in the art, the intensity of Fresnel reflection is governed by the difference in the index of refraction across a particular surface. If the outer medium is air, the difference in the index of refraction is about 0.5. Because the index of refraction of a typical paint medium is roughly equal to that of water, very little light is scattered when the inner surface of the smoke detector is covered with water. Some light is reflected by the outer surface of the water, but this light is more directional and smaller than the light scattered from the paint surface.
Accordingly, when the walls of the chamber become wet, the above-described process results in essentially complete absorption of all light emitted from the light source. During testing of the smoke detector, therefore, the detector will fail to receive any diffusely scattered light from inside the chamber and will send an erroneous fault signal indicating that the light source is not operational.
Thus, there is a need for a smoke detector that is capable of performing and being reliably tested in the presence of moisture, such as when carrying high humidity cargo, i.e., animals, fruit, flowers, or the like. In addition, such a smoke detector should be easy to manufacture and capable of being retrofitted into existing smoke detector locations.
These and other needs are provided, according to the present invention, by a smoke detector having a moisture compensating device that reflects a substantially constant percentage of diffusely scattered light regardless of moisture present on the surfaces of the smoke detector. The moisture compensating device includes a moisture-insensitive light trap, which absorbs a large percentage of the light incident thereupon, whether the surface of the light trap is wet or dry. The moisture compensating device also includes a moisture-insensitive reflector, which reflects a predetermined percentage of the light incident thereupon regardless of moisture on the reflector. Accordingly, the smoke detector of the present invention can be reliably tested even in instances which moisture has collected on the inner surfaces of the smoke detector since the moisture compensating device will still reflect a constant percentage of light, thereby avoiding the fault indication provided by conventional smoke detectors when the inner surfaces of the smoke detector become wet and alter the reflectivity of the surfaces thereof.
In particular, the smoke detector of the present invention includes a housing defining at least one opening for receiving smoke. The housing, however, does not permit light to enter from external sources. Instead, a light source is positioned inside the housing for emitting a light beam across at least a portion of the housing. A photodetector, such as a photodiode, is also positioned inside the housing for receiving diffusely scattered light from inside the housing. According to one embodiment, the detector and associated circuitry send a fault signal if the level of diffusely scattered light sensed by the detector falls below a minimum value, thereby indicating that the light source is no longer operable.
Advantageously, the smoke detector of the present invention also includes a moisture compensating device within the housing that can be at least partially illuminated by the light source. The moisture compensating device includes a light trap that is insensitive to the presence of moisture. In one embodiment, the light trap comprises a folded sheet of light-absorbing material, although many alternative configurations may also be used. The moisture compensating device also includes a reflector. The reflector is capable of reflecting a substantially consistent percentage of the light incident thereupon regardless of moisture present on the surface of the reflector. In one embodiment, the reflector is coincident with and, in some instances, attached to the light trap. The reflector can have many shapes and configurations, including a metallic strip or wire extending across a portion of the inside of the housing or light trap such that the light emitted from the light source is at least partially incident upon the reflector.
Thus, the smoke detector of the present invention overcomes the difficulties encountered by conventional smoke detectors by providing a moisture compensating device that reflects a substantially consistent percentage of light incident thereupon regardless of moisture present on the surfaces thereof. In effect, the smoke detector of the present invention reflects substantially the same percentage of light as a conventional light trap does when dry, regardless of any moisture whatsoever on the surfaces of the light trap of the present invention. Thus, the smoke detector can be reliably tested to insure proper operation of the light source, even in high moisture conditions. In addition, the smoke detector of the present invention is easy to manufacture and can be retrofitted into existing smoke detector locations in aircraft cargo bays and the like.